This invention relates to a process for controlling blown film thickness and the apparatus for performing the process; more particularly, the process and apparatus relate to a blown film thickness control for improved response time and accuracy, for significantly reducing scrap material.
Machines for producing plastic film by the blown film process operate generally on the principle of producing a plastic bubble by means of air interaction with molten plastic through a narrow annular gap. The gap is formed in a die, or more correctly, by the concentric positioning of two dies so as to provide an intermediate annular gap therebetween. The plastic bubble which is created by the machine is usually collapsed in a region above the die, by pulling the plastic film through a collapsing frame which has an "A" shape, wherein a gap at the top of the frame permits the film to be passed through the frame and over a series of rollers. At least one set of these rollers is referred to as a "nip" roller, and is driven by a motor which has a controllable rotational speed. The thickness of the plastic film may be controlled by varying the rotational speed of the motor, thereby varying the rotational speed of the haul-off rollers, and the linear movement of the plastic bubble. The film which is produced by this machine is then passed over a series of further rollers and edge cutters, and is ultimately collected on take-up reels for storage and disposition.
A continuing problem with machines of the type hereinbefore described is the problem of maintaining a film thickness within relatively narrow tolerances. Plastic films manufactured by these machines are typically made in film thicknesses ranging from 0.001 to 0.010 inch (1-10 mils), and it is desirable to control the thickness to .+-.3% or better. Film thickness is typically controlled by sensors which monitor the film as it is formed into the plastic bubble, and circuits which convert the sensor signals to motor drive signals to vary the speed of the haul-off rollers. The haul-off rollers may be increased in speed in order to produce a thinner film thickness, and decreased in speed in order to produce a thicker film thickness.
A further problem in this process is the problem of localized imperfections or variations in the die, which produces localized regions of thicker or thinner film. Any die which utilizes an annular gap for producing film will invariably have variations in gap dimension; a gap variation of only a few percent over a localized region is unacceptable, particularly when one considers that the film produced by the machine is collected on rollers, and a localized unevenness in the film will create a cumulative thickness as the take-up roll is filled, leading to a lumpy and unstable roll of the finished product. This problem has been at least partially solved by rotating the die as the machine is operated, to essentially distribute imperfections about the circumference of the bubble, so that a buildup of thicker or thinner film material will not distort the take-up roll. In a typical machine, the die is rotated at a very slow rate, usually at about 3-10 minutes per revolution. The blown film bubble moves at a linear speed in the range of 50-300 feet per minute, and therefore an imperfection in film thickness caused by the die gap will be distributed more or less evenly across the plastic film sheet which is ultimately placed on a take-up roll.
In compensating for film thickness variations, a thickness sensor placed against the outer surface of the bubble can provide an instantaneous measurement of thickness at its localized position. By virtue of the rotation of the die, a thickness sensor will eventually measure the film thickness of the entire bubble, because the film produced by each angular sector of the die will eventually move past the sensor. However, since the die is rotating at a relatively slow angular rate, a film thickness measurement of the entire circumference will require some 3-10 minutes of operation, during which time 100-3,000 feet of film may have been manufactured. If the overall average film thickness is measured to be outside of permissible limits, the rotational speed of the haul-off rollers can be controlled, but at the cost of producing a considerable amount of film outside of specifications. In the prior art, this problem has been addressed by mounting the sensor for measuring thickness on a circular trolley, and transporting the sensor about the bubble at the same time as the bubble is being formed. The rate of rotation of the sensor about the bubble is typically controlled at 1-3 minutes per revolution, thereby permitting a complete measurement of thickness of the entire bubble to be made within a much shorter period of time. While this does reduce the response time required by the system for correcting film thickness errors, it requires the additional expense of constructing the circular trolley system for the sensor. Further, since the relative movement of both the sensor and the plastic film bubble occurs while the sensor measurements are being made, measurement errors may be caused by the failure of the sensor to maintain close contact with the film bubble.
The present invention provides an improved apparatus and process for controlling the thickness of blown film, without the necessity for providing a rotating carriage for the film thickness sensor. Furthermore, the present invention provides a method for compensating for die gap variations, so that variations in the film thickness profile around the circumference of the bubble may be taken into consideration in generating the control signal for varying the speed of rotation of the nip rollers.